This invention generally relates to means for suppressing noise in an outboard marine engine. In particular, the present invention relates to means for suppressing noise transmitted from the exhaust housing of an outboard engine.
Typical marine engines are noisy, especially when being operated at higher rpm""s while driving a vessel rapidly through the water. This noisy operation is extremely unattractive to occupants of the vessel, as well as to passers-by, and it is highly desirable to reduce this noise without reducing vessel efficiency. Further, regulatory bodies, in their desire to improve the environment, are imposing emission standards on marine vessels. These standards not only regulate the contents of the emissions but also apply to the noise level of the emission. It is therefore highly desirable to provide a marine engine that is noise reduction efficient without detracting from the vessel operating efficiently.
More general than the noise reduction is noise control. Noise control requires an understanding of the vibro-acoustic behavior of the article in question with its environment. If boundary conditions permit, approximations can be made by isolating the article from its environment. This cannot be done xe2x80x9csimplyxe2x80x9d for an integrated structure. For example, an outboard marine engine is an integrated structure. To capture correctly the vibro-acoustic behavior of an outboard engine, the engine should be fully assembled, mounted to a boat and in the open water. For example, feedback from the added inertia of the water as the boat travels in the water could produce a narrow-band spectrum different from a steady-state condition. There is also feedback from the components of the engine, for example, the crankshaft and block can produce a phenomenon that does not exist for either part acting alone.
To determine the acoustic xe2x80x9cfingerprintxe2x80x9d for an integrated structure such as an outboard marine engine, a narrow-band analysis must be performed. This will allow identification of tones, i.e., frequency responses, of the interacting components. The components corresponding to these responses can be identified from the frequencies, i.e., based on wavelength and speed of sound. Vibro-acoustic treatments can be designed and or critically placed to attenuate or simply move a tone from one frequency to another. The effectiveness of this effort is based on the precision of the data and the methodology by which the data is acquired.
The precision of the data is a function of the frequencies of the data collected and of the transducer sensitivity. The frequency range of interest is a function of human hearing, i.e., 10 kHz is sufficient. For the present work, data was collected using accelerometers and microphones. Accelerometer data was collected to 5 kHz at 1 Hz bandwidth; microphone data was collected to 10 kHz at 2.5 Hz bandwidth. Acoustic intensity testing and stethoscopic probing both showed agreement that over 80% of the vibro-acoustic energy produced by a particular outboard marine engine was coming from below the interface between the engine""s upper and lower motor covers, a large part of the noise being transmitted from the exhaust housing. It was further discovered that a particular tone produced inside the exhaust housing did not change frequency as the rpm of the engine was modulated. This discovery led to the realization that a standing wave was being set up inside the exhaust housing, causing the exhaust housing to vibrate at low frequency (in one case, at about 3,500 Hz).
Thus there is a need for a structure which can be incorporated inside an exhaust housing of an outboard marine engine to break up standing waves and reduce noise output.
The present invention is directed to an improved exhaust housing having means for breaking up standing acoustic waves resonating inside the exhaust housing. Such standing waves intensify and prolong the acoustic noise transmitted from the exhaust housing. A standing acoustic wave can be produced when the passage through which air, e.g., exhaust gas, flows has a dimension which equals at least one fourth the speed of sound divided by the frequency of the standing wave.
In accordance with the preferred embodiment of the invention, this resonant condition is eliminated by incorporating plates inside the exhaust housing. These resonance-avoiding plates are generally parallel to the direction of flow from the powerhead and are welded to the walls of the exhaust housing. (The terms xe2x80x9cpowerheadxe2x80x9d and xe2x80x9cmotorxe2x80x9d will be used interchangeably throughout the written description and the claims.) The resonance-avoiding plates divide the exhaust housing into multiple channels, each channel having transverse dimensions smaller than the transverse dimensions of the unmodified exhaust housing. Consequently, any standing acoustic wave in one of the channels will have a frequency higher than the frequency of a standing wave in the unmodified exhaust housing. In addition, the plates serve to increase the stiffness of the exhaust housing, changing the mode of vibration of the exhaust housing from low to high frequency. As a result, the tones produced by the vibrating exhaust housing will be moved to higher frequencies, i.e., further away from the so-called Speech Interference Level 123 (SIL123) corresponding to the frequency range from 1,000 to 3,000 hertz.
The broad concept of the invention is directed to a boat propulsion system having a motor with an exhaust port for exhaust gases, the exhaust gases being exhausted via a resonance-avoiding exhaust housing. The exhaust housing is a walled enclosure having an inlet, an internal volume in flow communication with the inlet, and an outlet in flow communication with the internal volume. The exhaust housing further includes hollow structures for dividing a portion of the internal volume into a plurality of flow channels which extend in side-by-side relationship. The transverse dimensions of each flow channel is substantially less than the transverse dimensions of the walled enclosure. The result is that standing waves are shifted to a higher frequency range. The hollow dividing structures have internal volumes which communicate with space external to the exhaust housing via openings in the walled enclosure, which allow the admission of a cooling medium. The hollow structures also increase the stiffness of the walled enclosure of the exhaust housing, shifting the vibration mode of the exhaust housing to higher frequencies.
The invention further encompasses a method of retrofitting an engine exhaust housing comprising a walled enclosure having an inlet, an internal volume in flow communication with said inlet, and an outlet in flow communication with said internal volume. The retrofitting method comprises the step of dividing a portion of the exhaust housing internal volume into a plurality of flow channels which extend in side-by-side relationship. Each of the flow channels has an inlet which is closer to the exhaust housing inlet than the flow channel outlet is and an outlet which is closer to the exhaust housing outlet than the flow channel inlet is. The dividing step is accomplished by installing a hollow structure having an opening inside the internal volume of the exhaust housing, and forming an opening in the walled enclosure at a location such that the opening of the walled enclosure is in flow communication with the opening of the hollow structure. The installing step comprises the steps of attaching rigid plates to the walled enclosure such that the stiffness of the walled enclosure is increased.